Moulded bodies for use as bed supports or seat bases are found in a very wide variety of forms, such as mattresses and cushions, also seating in the home, or in means of transport. Such items are used not only by people, but also by animals.
What is common to all these moulded bodies is that the user feels a certain level of firmness. The force that a user exerts on the shaped body depends on his weight and his movements. At the same time the shaped body exerts a counterforce, where the system of user and shaped body pursues a state in which the force and counterforce are in equilibrium with one another. This state is called the quiescent state.
In most types of use the user is not completely stationary, but rather performs constantly changing movements that on each occasion require the adjustment to a new force equilibrium. In materials with a certain level of compliance, an alteration of shape occurs in reaction to the forces, or alterations in force, exerted by the user. The force that a user must exert so as to impress a shaped body is perceived by the user as firmness. If the material takes the form of a foamed material one also talks about compression hardness.
Estimation of the level of firmness as high or low can on the one hand be subject to subjective variability. The firmness of a shaped body can, however, also alter during the course of its lifetime of usage. Thus the original firmness of a shaped body can indeed be tested before purchase by sitting or lying on it. If the firmness of the shaped body alters in the course of time, the user has only a few options for correcting the firmness to the desired value. In the case of mattresses that are supported on slatted frames, for example, it is possible to move sliders on the slatted frame so as to adapt the firmness of the slatted frame. A further option of known art consists in altering the firmness of the shaped body by means of alterations of volume or density. For this purpose air or water bladders are normally used; these are introduced into or onto the shaped body.
What is common to all these procedures is that they require larger modifications and thus are very resource-intensive; they are therefore only carried out for alterations to the firmness that are meant to persist over longer time periods.